Iron is known to be an essential micronutrient for the photosynthetic growth of phytoplankton (algae) in global waters, particularly oceans. More than one-fifth of the world's oceans have iron-deficient surface waters with little or no algae growth despite the presence of all other essential growth nutrients (Kolber et al. Nature 371, 145 (1994)). These regions have been referred to as high-nitrate, low-chlorophyl (HNLC) areas. Martin and others have suggested adding iron to the ecosystems of these oceanic areas to encourage phytoplankton growth on a scale sufficient to affect the level of atmospheric carbon dioxide, which in turn might be of use to help prevent global warming. See, for example, Martin, J. H. et al. Nature 371, 123 (1994).
A major "iron-seeding" experiment performed in 1993 in an area of the equatorial Pacific Ocean currently devoid of algae showed that the addition of iron produced an immediate and dramatic growth of phytoplankton. The effect, however, was short-lived. A major reason was rapid loss of iron. Iron loss was attributed to iron sinking into deeper waters and/or possible adsorption of iron by organic material (Van Scoy, K. et al. New Scientist, p. 32 (Dec. 3, 1994), Martin, J. H. et al. Nature 371, 123 (1994)). Little effect on surface carbon dioxide concentrations was detected, presumably because of the short-lived nature of the experiment (Watson, A. J. et al. Nature, 371, 143 (1994)).
Wells, M. L. et al., in Nature, 353, 248 (1991), disclose that iron availability as a micronutrient for algae growth depends on its chemical liability and/or ease of dissolution. Natural sources of iron are often in an unconfined refractory particulate or colloidal form, unavailable for direct assimilation by phytoplankton. Such forms are eventually converted by natural means into bioavailable forms, but at a slow and unpredictable rate. Time-consuming chemical reactions must occur before this iron is "bioavailable", i.e., converted to a form which can be utilized in photosynthesis. Much "natural" iron is lost by sedimentation or other means before such conversion is complete. Johnson, K. S. et al., in Marine Chem. 46, 319 (1994), disclose that iron in particles or colloids must be at least partly solubilized for bioavailability to support phytoplankton growth. Experiments wherein iron addition led to rapid, albeit temporary, algae growth have employed soluble ferrous or ferric salts such as ferrous sulfate dissolved in acidified seawater (Van Scoy et al., supra).
Although the prior art has shown the positive impact on photosynthetic growth of adding nutrients such as iron, nutrient sources were supplied in a non-buoyant form. Non-buoyant sources quickly sank and disappeared from the growth sites, after which photosynthesis declined rapidly. Alternately, in order to mimic the major source of natural iron to remote areas of the world's oceans, atmospheric dust particles were added as aerosols. Such particulate sources of iron, even though mostly refractory (non-lablile) iron, contributed to photosynthetic phytoplankton growth, as measured by chlorophyl production. See K. S. Johnson et al., supra. It was found that photochemical reduction contributed to the bioavailablity of the particulate iron.
It is the purpose of the present invention to artificially provide micronutrients to phytoplankton on a more continuous or sustained basis. Floating or water-buoyant compositions containing photosynthetic growth micronutrients such as iron will remain longer in surface waters, and thus can increase the availability of nutrient iron for photosynthetic processes. Floating materials containing a source of one or more elemental nutrients, especially iron, on or near the surface of bodies of water deficient in such elemental micronutrients, will stimulate and maintain effective photosynthetic phytoplankton growth for a sustained period of time.